1. Field of the Invention
The present invention relates to an electrophotographic copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a fixing device for use in an image forming apparatus and including a fixing roller, a press roller, a heat roller having a heat source thereinside and a fixing belt passed over the fixing roller and press roller.
2. Description of the Background Art
A fixing device applicable to an image forming apparatus is disclosed in, e.g., Japanese Patent Application No. 2003-307963. The fixing device taught in this document is configured to achieve various purposes including reducing a warm-up time to a stand-by state and a buildup time from the stand-by state, speeding up fixation, stabilizing the output of a power supply, and reducing power consumption. The document proposes an image forming apparatus using such a fixing device also. A controller included in the fixing device controls the turn-on and turn-off of the individual heater connected to a power supply. More specifically, at the time of warm-up, buildup and printing, the controller turns on only the heater of the heat roller to thereby apply a sufficient amount of heat to the fixing belt, which is turning, via said heat roller. Further, in a stand-by state, the controller turns on the heaters of the heat roller and press roller in order to maintain the temperature of the heat roller and press roller. In this manner, the controller of the above fixing device turns on or turns off each heater at a particular timing.
Generally, in a conventional fixing device, a fixing roller and a press roller rotatable in pressing contact therewith each are provided with a surface layer formed of rubber, but a heat roller, accommodating a heater therein, is not provided with such a surface layer in order to have a small thermal capacity. While sharp thermal response is achievable if the heat roller is provided with a small thermal capacity small and if the heater of the heat roller is caused to generate a great amount of heat in a continuous sheet-pass mode, this scheme is unable to reduce power consumption. To reduce the power consumption of the fixing device and therefore the total power consumption of an entire image forming apparatus, the amount of heat to be generated by each of the heaters of the fixing roller and press roller is made smaller than the amount of heat to be generated by the heater of the heat roller.
However, the problem with the conventional fixing device stated above is that the heat roller, having a smaller thermal capacity than the fixing roller and press roller, is greatly effected by the temperatures of the fixing roller and press roller via the fixing belt at the beginning of a sheet-pass after the stand-by mode. More specifically, if the temperature of the fixing roller or that of the press roller is lowered in, e.g., the stand-by mode in which the fixing belt remains in a halt, the temperature of the fixing belt is low at the beginning of a sheet pass and therefore degrades fixation.
In light of the above, it is necessary to maintain the temperatures of the fixing roller and press roller above a preselected temperature in the stand-by mode. However, although the fixing roller and press roller both receive heat from the fixing belt via their surface layers at the time of start-up of the fixing device because the belt is turned, the temperatures of the fixing roller and press roller are lowered after the stop of movement of the belt partly because the amount of heat output from the heater is small and partly because the fixing roller and press roller each have a great thermal capacity.